LONDON (AP) – Children with attention deficit hyperactivity disorder are twice as likely to have missing or extra chromosomes than other children – the first evidence that the disorder is genetic, a new study says.

British researchers compared the genomes of 366 white British children from 5 to 17 years old with attention deficit hyperactivity, or ADHD, to those of more than 1,000 similar children without the disorder. The scientists focused on a sequence of genes linked to brain development that has previously been connected to conditions like autism and schizophrenia.

In children without ADHD, about 7 percent of them had deleted or doubled chromosomes in the analyzed gene sequence. But among children with the disorder, researchers discovered about 14 percent had such genetic alterations. Scientists also found that 36 percent of children with learning disabilities in the study had the chromosomal abnormalities.

“This is the first time we’ve found that children with ADHD have chunks of DNA that are either duplicated or missing,” said Anita Thapar, a professor at the MRC Centre in Neuropsychiatric Genetics and Genomics at Cardiff University who was one of the study’s authors.

She said the findings are too early to affect diagnosis or treatment and are only applicable to people of European Caucasian descent because studies have not been done yet on other ethnicities.

The condition is estimated to affect millions of children around the world, and scientists have long thought the disorder has a genetic component.

U.S. experts estimate that ADHD affects from three to five percent of school-age children in the United States. There are no figures for developing nations.

The study was paid for by Action Research, Baily Thomas Charitable Trust, the Wellcome Trust, Britain’s Medical Research Council and the European Union. It was published online Wednesday in the medical journal Lancet.

Peter Burbach, a professor of molecular neuroscience at University Medical Center Utrecht in the Netherlands, was surprised some of the genetic defects found for ADHD were identical to ones for autism and schizophrenia. He was not connected to the Lancet research.

“There’s a great chance the environment is modifying these genes,” Burbach said, adding the genes could lead to several brain disorders, depending on things like the child’s upbringing and other genetic factors.

He also thought scientists might eventually be able to reverse ADHD.

“This is not a structural abnormality in the brain, it’s just the last phase of development that’s gone wrong,” he said. “It could be the brain just needs to be fine-tuned.”

Philip Asherson, a professor of molecular psychiatry at the Institute of Psychiatry at King’s College London, said the study only dealt with a subset of people with ADHD and said the environment should still be considered a cause. In the case of some Romanian orphans, Asherson said there was proof that severe deprivation at an early age can lead to ADHD or other neurological problems.

Asherson said the medical world was still years away from being able to correct ADHD.

“The study doesn’t tell us a lot about what’s going on in the brains of people with ADHD,” he said. “If we can find out more about these genes and how they affect brain development, that may give us inroads, but it’s hard to say when that will be.”